April 16, 2010

With changing weather patterns and the start of the planting season, a look at the soil moisture status, probabilities of a late spring freeze, and potential El Nino impacts seems warranted.

Soil Moisture

If we look at this year’s conditions, El Nino continues to weaken, but is expected to have an impact into early June. Based on past El Nino events that continued into May, there is a strong tendency for above normal moisture across the eastern two-thirds of the Nebraska. If this occurs, planting delays are likely as current soil moisture levels won’t be able to store additional moisture. Recent temperature and wind patterns have promoted surface drying, but until plants emerge and begin using stored soil water, profiles will remain close to field capacity.

Historical Statistical Clues

Does the El Nino outlook mean that corn might be at risk for freeze damage once again this year? Not necessarily. Based on the past 15 El Nino events, I looked at the Growing Degree Day accumulations (Base 50/86) that accumulated up to the first fall hard freeze date (28°F) at Omaha using May 1, May 10, and May 30 emergence dates. All 15 events accumulated at least 2850 GDDs for a May 1 emergence, 2640 GDDs for a May 10 emergence, and 2430 GDDs for a May 30 emergence.

This analysis includes the 1992 season which was the coldest on record. Throw out that year and the minimum GDDs accumulated before a hard fall freeze was 3210 GDDs for a May 1 emergence, 2970 GDDs for a May 10 emergence, and 2710 GDDs for a May 30 emergence. These numbers were greatly influenced by above normal temperatures from July through September.

In the eastern third of Nebraska 12 of the 15 events had below average June precipitation, with six events receiving less than 50% of normal moisture. For the western two-thirds of the state 9 of the 16 events were below normal, with four failing to receive 50% of normal moisture.

July and August precipitation patterns were essentially a flip of the coin across the state. However, for those times when precipitation failed to reach average, 50% of the events had less than 50% of normal moisture. Couple these statistics with below normal June moisture and above normal July through September temperatures and the growing season drought risk increases to a 1 in 4 chance versus the historical trend of 1 in 7.5.

History indicates that drought risk is greater than normal across Nebraska during growing seasons following an El Nino event. This may be a year to pay particular attention to the eastern Corn Belt for drought development because history has shown that the dry conditions over southern Canada and the southeastern U.S. can intensify, merge, and build toward Nebraska as they did in 1983 and 1988.

Al Dutcher
Extension State Climatologist